Explicit segment search should fallback to default segment

[kjac]

When searching for a specific segment, we need to perform fallback to the default segment instead of only returning hits for that segment.

Unfortunately this likely means that we'll have to index segment fields on the default language document instead of having explicit documents per segment.

This will make segment querying a little more convoluted, and will raise the risk of us running into the upper field limitation of Examine. On the upside we'll have less documents in the index.